Environmental pollution caused by combustion-generated NO.sub.x emissions, is a matter of great concern to the public, and as well to industrial fuel users. Beginning in the 1960's, governmental agencies, indeed prompted by public concern with increasing levels of smog and air pollutants, imposed NO.sub.x reduction requirements upon existing power plants in major metropolitan areas. Industry, accepting the challenge, has already developed a large variety of technologies to meet the new needs. Modifying the combustion process has become the most widely used technology for reducing combustion generated NO.sub.x. In addition, a number of flue gas treatment technologies have been developed and are emerging as the primary method of control for certain applications, but have seen limited use where natural gas is the fuel of choice.
Oxides of nitrogen (NO.sub.x) are formed in combustion processes as a result of thermal fixation of nitrogen in the combustion air ("thermal NO.sub.x "), by the conversion of chemically bound nitrogen in the fuel, or through "prompt-NO.sub.x " formation. In addition to generating "thermal NO.sub.x ", i.e., by high temperature combination of free nitrogen and oxygen, where the fuels employed by such users (e.g. coal gas) contain substantial quantities of chemically bound nitrogen, certain combustion conditions will favor the formation of undesirable NO-type compounds from the fuel-bound nitrogen. "Prompt NO.sub.x " refers to oxides of nitrogen that are formed early in the flame and do not result wholly from the Zeldovich mechanism. Prompt-NO.sub.x formation is caused by 1) interaction between certain hydrocarbon components and nitrogen components and/or, 2) an overabundance of oxygen atoms that leads to early NO.sub.x formation. For natural gas firing, virtually all of the NO.sub.x emissions result from thermal fixation, which is commonly referred to as "thermal NO.sub.x ", or from prompt NO.sub.x. The formation rate is strongly temperature dependent and generally occurs at temperatures in excess of 1800.degree. K. (2800.degree. F.) and generally is more favored in the presence of excess oxygen. At these temperatures, the usually stable nitrogen molecule dissociates to form nitrogen atoms which then react with oxygen atoms and hydroxyl radicals to form, primarily, NO.
In general, NO.sub.x formation can be retarded by reducing the concentrations of nitrogen and oxygen atoms at the peak combustion temperature or by reducing the peak combustion temperature and residence time in the combustion zone. This can be accomplished by using combustion modification techniques such as changing the operating conditions, modifying the burner design, or modifying the combustion system.
Of the combustion modifications noted above, burner design modification is most widely used. Low NO.sub.x burners are generally of the diffusion burning type, designed to reduce flame turbulence, delay the mixing of fuel and air, and establish fuel-rich zones where combustion is initiated. Manufacturers have claimed 40 to 50 percent nominal reductions, but significant differences in the predicted NO.sub.x emissions and those actually achieved have been noted. The underlying cause for these discrepancies is due to the complexity in trying to control the simultaneous heat and mass transfer phenomena along with the reaction kinetics for diffusion burning. In addition, it is extremely difficult to obtain representative samples from the flame envelope of this type of burner, which when analyzed, can provide the necessary data to improve predictive models.
Illustrative of the foregoing and related techniques for NO.sub.x reduction, are the disclosures of the following United States patents:
DeCorso, U.S. Pat. No. 4,787,208 discloses a low-NO.sub.x combustor which is provided with a rich, primary burn zone and a lean secondary burn zone. NO.sub.x formation is inhibited in the rich burn zone by an oxygen deficiency, and in the lean burn zone by a low combustion reaction temperature. Ceramic cylinders are used at certain parts of the combustion chambers.
Fanuyo et al, U.S. Pat. No. 4,731,989 describes a combustion method for reducing NO.sub.x emissions, wherein catalytic combustion is followed by non-catalytic thermal combustion.
Davis, Jr. et al, U.S. Pat. No. 4,534,165 seeks to minimize NO.sub.x emissions by providing operation with a plurality of catalytic combustion zones and a downstream single "pilot" zone to which fuel is fed, and controlling the flow of fuel so as to stage the fuel supply.
DeCorso, U.S. Pat. No. 4,112,676 shows a combustor generally of the diffusion burning type for a gas turbine engine.
Pillsbury, U.S. Pat. No. 4,726,181 provides combustion in two catalytic stages in an effort to reduce NO.sub.x levels.
Kendall et al. U.S. Pat. No. 4,730,599 discloses a gas-fire radiant tube heating system which employs heterogeneous catalytic combustion and claims low-NO.sub.x catalytic combustion.
Shaw et al, U.S. Pat. No. 4,285,193 describes a gas turbine combustor which seeks to minimize NO.sub.x formation by use of multiple catalysts in series or by use of a combination of non-catalytic and catalytic combustion.
Pfefferle, U.S. Pat. No. 3,846,979 describes low NO.sub.x emissions in a two-stage combustion process wherein combustion takes place above 3300.degree. F., the effluent is quenched, and the effluent is subjected to catalytic oxidation.
Beremand et al, U.S. Pat. No. 4,087,962, discloses a combustor which utilizes a non-adiabatic flame to provide a low emission combustion for gas turbines. The fuel-air mixture is directed through a porous wall, the other side of which serves as a combustion surface. A radiant heat sink is disposed adjacent to the second surface of the burner so as to remove radiant energy produced by the combustion of the fuel-air mixture, and thereby enable operation below the adiabatic temperature. The inventors state that the combustor operates near the stoichiometric mixture ratio, but at a temperature low enough to avoid excessive NO.sub.x emissions. In one embodiment the radiant heat sink comprises a further porous plate.
In U.S. Pat. No. 4,811,555, of which Ronald D. Bell, one of the applicants of the present application, is patentee, there is described a cogeneration system in which NO.sub.x is controlled by the treatment of the turbine exhaust by a combination of combustion in a reducing atmosphere and catalytic oxidation.
In McGill et al, U.S. Pat. No. 4,405,587, for which Ronald D. Bell is a co-patentee, the NO.sub.x content of a waste stream is controlled by treating it and subjecting it to high-temperature combustion in combined reducing and oxidation zones.
Recent work by several of the present co-inventors and others, has resulted in a combustion device which utilizes a highly porous inert media matrix to provide for containment of the combustion reaction within the porous matrix--which may comprise fibers, beads, or other material which has a high porosity and a high melting temperature. Preferably, a ceramic foam is used. This ceramic, sponge-like material has a porosity (typically about 90%) which provides a flow path for the combustible mixture. The energy release by the gas phase reactions raises the temperature of the gases flowing through the porous matrix in the post-flame zone. In turn, this convectively heats the porous matrix in the post-flame zone. Because of the high emissivity of the solid in comparison to a gas, radiation from the high temperature postflame zone serves to heat the preflame zone of the porous material which, in turn, convectively heats the incoming reactants. This heat feedback mechanism results in several interesting characteristics relative to a free-burning flame. These include higher burning rates, higher volumetric energy release rates, and increased flame stability resulting in extension of both the lean and rich flammability limits. In addition to the ability to achieve very high radiant output from a very compact combustor, flame temperature increases are negligible. This is an important consideration with respect to NO.sub.x control purposes.
A one-dimensional mathematical model was constructed that included both radiation and accurate multi-step chemical kinetics. This model was used to predict the flame structure and burning velocity of a premixed flame within an inert, highly porous medium. The various predictions of this model have been discussed by Chen et al. See "The Effect of Radiation on the Structure of Premixed Flames Within a Highly Porous Inert Medium", Y-K Chen, R. D. Matthews, and J. R. Howell; Radiation, Phase Change, Heat Transfer, and Thermal Systems. ed. by Y. Jaluria, V. P. Carey, W. A. Fiveland, and W. Yuen (eds.), ASME Publication HTD-Vol. 81, 1987. "Premixed Combustion in Porous Inert Media"; Y-K Chen, R. D. Matthews, J. R. Howell, Z-H Lu, and P. L. Varghese, Proceedings of the Joint Meeting of the Japanese and Western States Sections of the Combustion Institute, pp. 266-268, 1987; and "Experimental and Theoretical Investigation of Combustion in Porous Inert Media", Y-K Chen, R. D. Matthews, I-G Lim, Z. Lu, J. R. Howell, and S. P. Nichols Paper PS-201, Twenty-Second Symposium (International) on Combustion, 1988. These papers demonstrate that a porous matrix (PM) combustor can provide a number of advantages over diffusion burners. However, these papers are focused on the development of this new concept, but are not concerned with the problem of NO.sub.x emissions, much less with the effective reduction of same.